Probing the Distribution of Alkali Cations at the Au/Electrolyte Interface with Vibrational Spectroscopy

The electrode/electrolyte contact represents a material interface whose catalytic properties are determined by the interaction of the solid and liquid sides. This interaction gives rise to the electrochemical double layer (EDL), which forms the reaction environment of electrocatalytic processes. Therefore, understanding how the structure of the EDL emerges from the coupling between the solid and liquid phases is central to the development of efficient energy storage and conversion devices. Of particular interest is the distribution of cations of the supporting electrolyte in the EDL; it has been found that cations of the supporting electrolyte influence numerous electrocatalytic processes. However, our understanding of the distribution of these cations at the electrode/electrolyte interface is still poorly understood due to numerous experimental challenges in probing (alkali) metal cations at electrocatalytic interfaces. In this study, we use operando surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the distribution of alkali cations in the EDL. We demonstrate that the asymmetric CH₃ deformation mode of tetramethylammonium (methyl₄N⁺) is a powerful probe of the distribution of alkali cations in the EDL. The vibrational probe simultaneously characterizes the populations of alkali cations in the inner and outer Helmholtz planes. We found that Cs⁺ has a higher<br/>tendency to accumulate in the inner and outer Helmholtz planes of the EDL compared with Li⁺ under CO₂ -to-CO conversion conditions. We rationalize these findings in terms of the hydration free energies of the cations and electrostatic interactions with the electrode. These observations contribute to a deeper understanding of the electrode/electrolyte material interface and how interfacial properties can be<br/>tailored for energy storage and conversion.